Prosthetic implant and assembly method

ABSTRACT

An orthopedic prosthetic implant comprises a metal alloy stem element ( 13, 63, 113 ), which has one end portion ( 19, 69, 119 ) constructed to reside in the medullary cavity of a bone and an integral connector ( 23, 73, 123 ) at the opposite end to which crystalline brittle head ( 17, 67, 117 ), preferably made of pyrocarbon-coated graphite, is joined. The head interfaces with human bone, and its effective joinder to the stem element is achieved through a polymeric insert ( 15, 65, 115 ) of proportional shape and design which has selected elastic properties. The design and material of the polymeric insert allow it to be securely received within an interior cavity ( 35, 77, 131 ) of the pyrocarbon-coated graphite head and mated to the stem connector in an either rigidly or bi-polar arrangement. The method of joinder allows the construction of composite implants that utilize the most desirable properties of metallic and brittle crystalline materials.

This application is a division of U.S. Ser. No. 12/233,976, filed Sep.19, 2008, which is continuation of International Application No.PCT/US07/64594, filed 22 Mar. 2007, which claims priority from U.S.Provisional Application Ser. No. 60/743,661, filed Mar. 22, 2006, thedisclosures of which are incorporated herein by reference.

This invention relates to prosthetic bone implants, and moreparticularly to a prosthetic bone implant to be used at a biologicaljoint, and to methods for making such implants. Still more particularly,the invention relates to a prosthetic implant wherein a metal stem isjoined to a head made of brittle crystalline material and to suchassembly methods.

BACKGROUND OF THE INVENTION

The field of prosthetic implants to treat conditions of fracture,arthritis and other such conditions has grown greatly in the past 2-3decades, and much work continues in these areas. Pyrocarbon-coatedgraphite materials have proved to be extremely wear-resistant andbiocompatible, and they have become the materials of choice for certainapplications where strength and other parameters can be met. U.S. Pat.Nos. 5,645,605, 6,159,247, 6,217,616, and 6,699, 292 and PublishedPatent Application No. 2005/0033426 are examples of prosthetic implantsthat can be used at biological joints in the human body or the like.Although these patents illustrate the use of integral structures forsuch implants, there is also interest in constructing bone implants,particularly those having an articular head, with a biocompatible metalalloy stem and a head of brittle crystalline material, such aspyrocarbon-coated graphite. Such a combination is considered to havecertain advantages because the properties of pyrocarbon can be tailoredto more closely match properties of bone where an interface will occur,generally at an articulating surface. However, the differences betweenthe structural properties of metal alloy stems and pyrocarbon-coatedgraphite heads pose a problem in designing such implants that can beeffectively assembled and will have long lifetime. U.S. Pat. No.6,997,958 recognizes the problem and proposes to limit the amount oftensile stress that may be applied to a head of brittle material when aMorse taper connection is employed; however, such a solution leaves thebrittle head subjected to residual stress throughout its lifetime whichmay not be desirable. As a result, other solutions to this problem havecontinued to be sought.

SUMMARY OF THE INVENTION

A prosthetic implant for implantation at a biological joint utilizes anintegral, one-piece polymeric insert to join a metal stem having aconnector at one end to a head made of brittle crystalline material,particularly pyrocarbon-coated graphite. The head is formed with anentrance to an interior cavity of a size such that at least a portion ofthe polymeric insert must be elastically deformed to completely enterthe cavity. The insert also has a cavity with an entrance region that issmaller than a corresponding dimension of an integral connector providedat the end of the metal alloy stem. The proportioning of the componentsis such that the polymeric insert preferably achieves an interferencefit within the cavity of the pyrocarbon-coated head when mated, and theproperties and dimensioning of the insert are such that it can alsoaccommodate small tolerances in the thickness of a pyrocarbon-coatedinterior cavity of the head and yet produce a strong compositesubassembly. The properties of the polymeric insert are chosen toaccommodate the entry of the connector through the smaller entranceregion by elastically deforming Depending upon the ultimate arrangementdesired, the proportioning may be such that the insert, upon return toits original physical configuration, may result in an interference fitat certain juxtaposed surfaces. Generally, the design will be such that,during assembly, the elastic limit of the polymeric material will not beexceeded so no significant plastic flow will occur; to facilitate thefinal mating of a subassembly of two components with the remainingcomponent, i.e. either the metal alloy stem element or the brittlecrystalline head, a circumferential relief region is provided. The stemcan be designed with a neck of a specific length so that the head willeither (a) seat on a flange that is a part of the stem or (b) pivot overa desired length of arc on a spherical connector at the end of the stem.

In a particular aspect, the invention provides a prosthetic implant forimplantation into a resected bone, which implant comprises a metal stemelement which has a connector at one end that is shaped with a reentrantregion of reduced dimension, an integral one-piece polymeric insertwhich has a central cavity that receives said connector, and a headformed of brittle crystalline material having a central cavity beingproportioned to receive said insert, said head cavity having an entranceof a size such that at least a portion of said polymeric insert mustelastically deform inward to enter said cavity, said insert centralcavity being formed with means for interengaging with said connectorwhich requires radially outward deformation of at least a portion ofsaid insert to lock said insert and said connector in engagement, andmeans providing a circumferential relief region into which a portion ofsaid polymeric insert can elastically deform, and said integralpolymeric insert being made of polymeric material having an elasticity,such that (a) it can deform radially inward sufficient to facilitate itsentry into said head cavity and then return to shape, (b) it can deformradially outward to facilitate assembly with said connector and thenreturn to shape, and (c) once assembled with both said head and saidstem connector, disassembly cannot inadvertently occur, with said finalassembly being facilitated by said relief region location.

In a more particular aspect, the invention provides a prosthetic implantfor implantation into a resected bone at a joint, which implantcomprises a metal stem element which has a connector at one end that isshaped with a reentrant region of reduced dimension, an integralone-piece polymeric insert which has an interior cavity that receivessaid connector and a flange that circumscribes an entrance to saidcavity, and a head having an exterior articular surface and an interiorcavity proportioned to receive said insert, said head being formed froma graphite substrate having interior and exterior pyrocarbon surfaces,said head cavity having an entrance formed by a reentrant entrance lipof a lesser inner diameter and a size such that said polymeric insertmust elastically deform radially inward to enter said cavity, saidpolymeric insert cavity having an entrance region of a size smaller thansaid connector and having an outer groove which receives said headentrance lip and is proportioned to provide an annular gap in saidgroove, and said integral polymeric insert being made of polymericmaterial having an elasticity, such that (a) it can deform radiallyinward sufficient to facilitate its entry into said head cavity andreturn to shape, (b) its entrance region can deform radially outward tofacilitate assembly with said connector, and (c) once assembled withboth said head and said stem connector, disassembly cannot inadvertentlyoccur.

In another particular aspect, the invention provides a method forforming a prosthetic implant, which method comprises providing a metalalloy stem element which has a connector at one end that is shaped witha region of reduced diametric dimension, providing a head ofcrystalline, brittle material having a cavity formed with an entrance ofreduced diameter, providing an integral polymeric insert that isproportioned to seat within said cavity in said head, which insert hasan interior central cavity that is proportioned to receive saidconnector at the end of said stem element and to interengage therewithat said connector region of reduced diametric dimension, forming asubassembly by mating said polymeric insert with said head by insertionof said insert through said entrance into said head cavity in a mannerin which the polymeric material deforms elastically radially inward tofacilitate its entry and returns to shape within said head cavity, andthen completing said prosthetic implant by mating said subassembly withsaid stem element by inserting said connector into said interior cavitywithin said polymeric insert by causing said polymeric material toelastically deform radially outward at said interengaging means where itis accommodated by a circumferential relief region and then to return toa configuration having an interior diameter which thereafter preventsinadvertent disassembly of said head subassembly from said stem element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of the components of a prostheticimplant embodying various features of the invention.

FIG. 2 is an exploded view of the components of FIG. 1 in cross section.

FIG. 3 is a cross-sectional view similar to FIG. 2 showing a subassemblyof the head and insert components.

FIG. 3A is an enlarged fragmentary view of a portion of the subassemblyshown in FIG. 3.

FIG. 4 is a cross-sectional view of the complete assembly of the threecomponents of FIG. 1.

FIG. 5 is an exploded perspective view of the three components of analternative embodiment of a prosthetic implant embodying variousfeatures of the invention.

FIG. 6 is a view showing the exploded components of FIG. 5 incross-section.

FIG. 7 is a cross-sectional view of the assembled three components ofFIG. 5.

FIG. 8 is a cross-sectional view similar to FIG. 7 with the head pivotedwith respect to the stem.

FIG. 9 is an exploded perspective view of three components of a furtheralternative embodiment of a prosthetic implant embodying variousfeatures of the invention.

FIG. 10 is an assembled view of the components of FIG. 9 shown incross-section.

FIG. 11 is a fragmentary cross-sectional view enlarged in size of aportion of FIG. 10 illustrating the interengagement between thecomponents that make up the prosthetic implant.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Illustrated in FIGS. 1 through 4 is a prosthetic implant 11 designed toreplace the proximal end portion of the radius. The implant 11 consistsof three components: a metal alloy stem element 13, a plastic insert 15and a head 17 of a brittle, crystalline, nonmetallic material,preferably a pyrocarbon-coated graphite substrate. When it is necessaryto replace the proximal head of the radius for whatever reason, theimplant provides an axisymmetric prosthesis similar to that shown inU.S. Pat. No. 6,217,616, entitled Elbow Prosthesis. By brittle is meanta crystalline material that fractures or fails instead of undergoingpermanent deformation; such materials, although strong in compression,are inherently weak in tension.

The stem element 13 includes a stem portion 19 that is proportioned tobe received in the medullary cavity of the resected radius. The stemportion may have any suitable cross-section, e.g. conical, cylindrical,polygonal or splined; it is designed to resist rotation relative to theradius once implantation has taken place. The radial axis is defined asthe axis which passes through the proximal and distal heads of theradius. The stem element 13 is formed with a thin, circular flange 21extending radially outward therefrom at a location intermediate itsends; the flange 21 defines the end of the implanted stem portion 19.When implanted, the undersurface of the flange 21 seats against the endsurface of the resected radius. A connector 23 is constructed at the endof the stem, which is separated from the flange 21 by a neck 25 ofreduced diameter. Although the flange 21, the connector 23 and the neck25 are all circular in cross-section (when viewed in planesperpendicular to the axis of the stem), they could, if desired, be ofpolygonal or other suitable cross-section. The neck 25 thus provides areentrant region between the flange 21 and the undersurface of theconnector 23.

The stem element 13 is preferably machined from a strong, biocompatible,metal alloy. Materials such as titanium, stainless steel, andcobalt-chrome-molybdenum alloys that are biocompatible may be used. Suchmaterials have the strength desirable to provide a strong replacementimplant at a joint or the like where it will be subject to stresses.

The head 17 at the end of the implant will interface with the patient'snative bones, and is made of a crystalline, nonmetallic material, e.g. aceramic, which is inherently brittle. Although alumina and zirconiaceramics are very useful for many applications, it has been found that adense pyrocarbon surface has superior properties for such an implantwith an articular interface, and such is desirable and preferred. Adense, isotropic graphite substrate 31 that is coated with a uniformlayer 33 of pyrocarbon continuously about its entire exterior surface isfound to provide excellent performance The pyrocarbon coating 33 shouldbe at least about 200 microns thick for surfaces that will be subject towear, preferably at least about 400 microns thick, and more preferablybetween about 500 and about 1000 microns thick. For surfaces where wearis not a factor, for example within a cavity where there is no relativemotion, a coating thickness of about 50 microns or more should beadequate. During articulation of the elbow, the axial end of the head 17of the illustrated insert 11 slides on the capitulum during flexion andextension of the elbow, and it generally rotates on the capitulum duringpronation and supination of the forearm and hand. Moreover, the contactwith the capitulum resists valgus forces applied to the arm, and alsoresists axial loads transmitted from the wrist to the elbow resultingfrom the gripping function of the hand. It is for this purpose that theaxial end of the head 17 is provided with a shallow concave surface 27.

The lateral surface of the radius head is received in the radial notchthat is formed in the medial portion of the ulna. The head 17 is formedwith a generally barrel-shaped exterior surface 29 to interface well atthe radial notch of the ulna. The head 17 of the radius is retained inthis location at the elbow by the radial collateral ligament and theannular ligament of the radius. The annular ligament is attached to theulna and is supported by the collateral ligament, which is in turnattached to the humerus, extending from a lateral region of thecapitulum and being disposed about the head and over the annularligament.

It has been found that pyrolytic carbon-coated, graphite substrates canbe used to create prostheses having a modulus of elasticity within about150% of the modulus of elasticity of natural bone; thus, this isconsidered to be a preferred material for manufacturing such prostheses.A particular pyrocarbon which is being marketed as On-X carbon (see U.S.Pat. No. 5,641,324) has advantageous properties for use in orthopedicprostheses such as these, particularly when such is coated upon asubstrate of isotropic, fine grain graphite. The result is the creationof a strong radial component prosthesis which has excellentbiomechanical properties. Because pyrocarbon is both physiologicallyinert and biochemically compatible with bone, and because the elasticmodulus of such a pyrocarbon-coated graphite substrate is very close tothat of cortical bone, such a prosthesis is highly biomechanicallycompatible and may be effectively used in such orthopedic implants,particularly those at joints within the human body where its articularsurface is important. In addition to its highly compatible modulus ofelasticity, pyrocarbon, and particularly On-X carbon, illustratesexcellent wear characteristics at its interface with bone and also withcartilage, resulting in an implant which is highly bone compatible.

Consistent with the foregoing, the head 17 is made from a machinedsubstrate of isotropic graphite 31 that is then coated with pyrocarbonin a fluidized bed coating apparatus, as known in this art, so as toprovide a continuous pyrocarbon surface 33 about its entire exterior, asbest seen in FIG. 2. To unite the pyrocarbon-coated graphite head 17 tothe metal alloy stem 13 without the creation of substantial undesirableresidual stresses, a carefully designed polymeric insert 15 is used,which is received in an interior central cavity 35 located in the head17. The cavity 35 is axially aligned and includes an interiorcylindrical wall 37 which is a surface of a right circular cylinder. Thecavity 35 has an entrance 39 in the form of a reentrant arcuate lip ofat least about 10% smaller diameter; it is designed to capture theperipheral portion of the polymeric insert 15 therewithin in a pocketcreated by the lip. Thus, the head 17 is very effectively joined to theinsert 15, which is thereafter mated to the connector 23 at the end ofthe stem element 13.

The employment of a metal alloy stem for seating in the medullary cavityof the resected radius (or other such bone) and a pyrocarbon-coatedgraphite head for interfacing with and articulating with adjacent bones,e.g. the ulna and the humerus, allows one to take advantage of thepreferred mechanical properties of both structural materials so long asa satisfactory arrangement mating the metallic stem and the crystallinehead can be effectively and efficiently provided. It is in this respectthat the polymeric insert 15 is designed and used; it is designed totake into consideration the relative stiffness and Young's Modulus ofeach of these two diverse materials, and particularly the brittleness ofisotropic crystalline graphite, and effectively mate the two materials.Dimensioning is such that a residual strain on the head 17 of less than10% of fracture strain can be achieved. Moreover, it can be appreciatedthat the pyrocarbon coating of graphite substrates which is carried outin a fluidized bed coater, such as that taught in U.S. Pat. No.6,410,087, requires precise control, and coating both the exterior andthe interior surfaces in a component, such as the graphite substrate 31for the head 17, poses particular problems from the standpoint oftolerances. The thickness of the coating of the exterior surfaces thatwill be subject to wear is regulated to achieve the preferencespreviously set forth; the coating may be of somewhat lesser thicknessupon interior surfaces that are not subject to wear. It has been found,however, that the use of an ultrahigh molecular weight polyethylene(UHMWPE) of a density of about 0.94 gm/cm³, and meeting ASTM StandardF648, allows an insert to be designed that will effectively join one tothe other and create a strong interference fit between the two withoutrequiring close coating tolerances to be held for the interior wallsurfaces of the cavity 35 in the graphite substrate and while stillrespecting the brittleness of graphite. Such UHMWPE will have a tensilemodulus of about 100,000 psi; it will prevent subsequent disassembly,particularly when the desired interference fit is achieved byappropriately proportioning the components.

As best seen in FIG. 2, the polymeric insert is formed with an exteriorlateral surface or wall 41 that is shaped to juxtapose with and, ifdesired, create a low stress interference fit against the interior wallsurface 37 in the pocket within the pyrocarbon head 17. The upper end ofthe lateral wall of the insert is chamfered to provide a tapered surface43, and the lower exterior region of the insert is constructed with aneck defined by a circumferential groove 45 located just above acircular bottom flange 47, which has a flat undersurface that isdesigned to juxtapose with the upper surface of the flange 21 of thestem element, seating fairly tightly against it if desired. In theillustrated construction, the inward protruding entrance lip 39 to thecavity 35 in the pyrocarbon head 17 is a convex surface section of atorus, and the exterior circumferential groove 45 in the insert is asimilarly shaped section of a concave toroidal surface providing ahollow region. The dimensioning is such that an annular gap 48 ofcrescent shape in cross-section is provided between the facing toroidalsurfaces in the head/insert subassembly, which serves as a relief regionas described hereinafter. In the illustrated embodiment, the upper endof the insert has an optional opening 49 at the top; however,alternatively, its upper end it could be closed if desired.

The polymeric insert 15 has an interior cavity 51 that is designed toreceive and mate with the exterior surface of the connector 23 at theend of the stem element. In the embodiment shown, the insert cavity 51has an interior, right circular, cylindrical surface 53 proportioned tomate with the surface of the same shape that forms the lateral exteriorof the connector 23. The polymeric insert 15 is formed with an entranceregion 55 at its lower end having an upwardly and inwardly taperedsurface. The entrance region 55 terminates with an annular locking ring57 that will, when mated with the connector 23, interengage or seatagainst an annular undersurface 59 that is formed on the connector 23 ofthe stem element at the location where it meets the neck 25. Thediameter of the cavity 51 is preferably at least about 5% greater thanthe inner diameter of the locking ring 57 located at the upper end ofentrance 55. The specific properties of each of these components will bebetter understood through the following description as to how the insert11 is assembled from these three components.

The implant 11 is illustrated in FIG. 3 with the stem element 13 spacedfrom a subassembly wherein the insert 15 has been mated with thepyrocarbon-coated head 17. The subassembly depicted in FIG. 3 is simplyachieved by relative movement of the head 17 and the insert 15 along thecentral axis. There is initial engagement between the tapered chamfersurface 43 and the reentrant arcuate entrance lip 39 leading to thecavity of the head 17. The UHMWPE material chosen for the insert 15 hassufficient elasticity that allows it to be gradually radially inwardlycompressed as the hollow insert passes through the reduced circularregion of the entrance lip 39; the components are appropriately sized sothat the elastic limit of the polymer is not exceeded. Once in placewithin the pocket provided in the head cavity 35, the polymeric insertreturns to its original dimensions, and if desired, it may effect aninterference fit between the juxtaposed surfaces 41 and 37. The arcuateentrance lip 39 leading to the head cavity 35 is seated snugly in thehollow of the exterior groove 45 in the polymeric insert which pinchesagainst it along two axially spaced circular regions, as seen in FIG.3A. The proportioning is such to leave an annular gap or region 48 ofcrescent-shaped cross-section which serves a relief purpose describedhereinafter. This seating, along with any interference fit between thetwo cylindrical surfaces, locks the subassembly components in tightinterengagement. Interfitting and tight interengagement provides astrong composite head; the physical character of the polymeric materialof the insert combines with the physical character of pyrocarbon-coatedgraphite to provide a composite head which exhibits improved overallstrength and physical properties, compared to an inherently brittle,pyrocarbon-coated graphite head. The polymeric insert 15 canpreferentially absorb shock and insulate the head from possiblefracture, and it can also cushion loading at the joint that wouldotherwise directly stress the pyrocarbon-coated graphite.

Once the subassembly is completed, subsequent relative axial movement tointerengage the subassembly and the stem element 13 produces thecomplete assembly shown in FIG. 4. As these two components are movedinto engagement with each other, the peripheral, upper edge of theconnector 23 which is rounded, e.g. arcuate, contacts the taperedsurface entrance surface 55 of the insert 15, and as relative movementcontinues, the polymeric material is deformed radially outward fromforces acting in a radially outward direction, squeezing it toward thesurrounding annular lip portion of the confining pyrocarbon-coated head17. It is here that the value of the annular relief gap 48 plays itspart. To avoid the elastic limit of the UHMWPE polymer being exceeded inthe region of the locking ring 57, as a result of rigid confinementbetween the surrounding annular reentrant lip 39 of the head 17 and thetransiting cylindrical surface of the connector 23, the relief gap 48 issized to accommodate an adequate amount of elastic movement in the wallof the insert in this region and should be of a volume of at least about90% of that of the locking ring. This circumferential relief avoids anysignificant plastic flow of the polymer as the larger diameter connector23 passes therethrough which preserves the contour of the locking ring57 to assure an ultimate locking fit of it and the undersurface flange59 of the connector 23.

Once the passage is complete, the flat undersurface of the flange 47 atthe bottom of the insert has become juxtaposed with the upper surface ofthe circular flange 21 of the stem, and the connector 23 is seated inthe mating cavity 51 of the insert. In this position, the locking ring57, which has been preserved as a result of the presence of the reliefgap 48, is juxtaposed with the facing annular undersurface 59 of theconnector, where it meets the neck, so that the connector 23 of the stemcannot be withdrawn from the overall assembly without deforming thelocking ring portion of the entrance region of the polymeric insert. Toattempt disassembly, it would be necessary to grasp both the implantablestem portion 19 and the head 17 and try to axially pull them apart; itcan be seen that such movement would be strongly resisted by thejuxtaposed ring 57 and undersurfaces 59. The at least 10% difference indiameter between the reentrant lip 39 and the outer diameter of theinsert 15 prevents their disassembly without destruction of onecomponent of the subassembly. Thus, it can be seen that a very secureconnection is achieved.

Because the proportioning of the interior cavity 51 of the polymericinsert and the exterior surface of the connector 23 can be held to closetolerances, any desired relationship can be reasonably attained. Forexample, the dimensioning can be such, as described above, that aninterference fit is achieved between the lateral surface of theconnector 23 and the interior surface of the insert cavity 51.Alternatively, if it is desired to allow relative rotation of the head17 on the end of the stem 13, the proportioning could be such that therewould be sufficient clearance between the exterior surface of theconnector 23 and the interior facing surfaces of the polymeric insertcavity that rotational movement about the axis would be permitted. Inthis case, the entrance region 55 would be sized so that it would permitrotation about the interface between the entrance region flange 47 andthe flange surface 21 of the stem element 13.

Illustrated in FIGS. 5 through 8 is an alternative embodiment showing animplant 61 where the attitude of the head may be allowed to varyslightly with respect to the axis of the stem. The implant 61 againconsists of an assembly of three components, a metal stem element 63, apolymeric insert 65 and a pyrocarbon-coated graphite head 67. Asdescribed previously, the metal alloy stem element 63 includes animplantable stem portion 69, a circular flange 71 and a connector 73,which in this embodiment is a section of a sphere, that surmounts a neck75. The spherical surface of the connector 73 is at least about 10%greater than that of a hemisphere, and preferably at least about 40%greater and more preferably at least about 80% of the surface of asphere.

The head 67 is essentially the same as the head 17 describedhereinbefore; it is an isotropic graphite substrate that is coated witha continuous coating of pyrocarbon having a thickness of at least about200 microns across its entire exterior surface. The pyrocarbon coatingis continuous so as to cover the walls of its interior cavity 77, entryto which cavity is through a similar entrance having a reentrant lip 79.Similarly, the exterior lateral surface of the polymeric insert 65 isagain essentially the same as heretofore described. However, thispolymeric insert has a flat upper surface 81, which is juxtaposed withthe flat interior surface that forms the upper end of the cavity 77 inthe head, and it has a cylindrical, lateral wall 83, which is receivedin the pocket in the head and proportioned to juxtapose with and, ifdesired, form an interference fit within the mating, pyrocarbon-coated,interior lateral surface of the cavity 77. The polymeric insert 65likewise has a circumferential groove or hollow 85 which receives thereduced diameter reentrant lip 79 of the head, which groove 85 islocated just above a circular flange 87 at the bottom of the insert. Itis proportioned to leave an annular relief gap 88 similar to the gap 48of crescent shape cross-section.

The major difference lies in that the interior cavity 89 of the insertis spherical so as to mate with the spherical connector 73 at the end ofthe stem element 63. As best seen in FIG. 7, the cavity 89 again has anannular arcuate entrance 91 of reduced diameter leading into thespherical cavity; the cavity 89, if cut by an axial plane, would subtendan arc of at least about 220° and preferably at least about 240°, e.g.between about 240° and 250°, which assures locking the connector 73within the head subassembly. The annular entrance 91 of the insert isshaped to provide a tapered lead-in surface 92. It may be arcuate asillustrated, e.g. a section of the surface of a torus, or it may be asection of a cone. The circular edge where the entrance meets the cavity89 is preferably slightly rounded, e.g. a radius of about 0.020 in. (0.5mm) might be used.

A further difference in the construction of the implant 61 is that it isconstructed so as to be bi-polar, a term used in orthopedics to indicatethat the attitude of the head 67 can be varied relative to the axis ofthe implant. To achieve such construction, the stem element has a neck75 that is elongated, relative to the neck 25, so that, as seen in FIG.7, the undersurface of the flange 87 at the bottom of the insert isspaced apart from the flange 71 of the stem 63 when the head is alignedcoaxially with the stem. Moreover, the upper surface of the flange 71 onthe stem element is preferably shaped to have a peripheral frustoconicalsurface region 93.

Accordingly, the head 67 of the insert 61, as a result of the sphericalconnector 73 and the elongated neck 75, can change in attitude bypivoting along an arc in any direction; moreover, it can rotate on theconnector. Generally, the amount of pivoting allowed, from the coaxialalignment shown in FIG. 7, in one direction in the axial plane of theimplant will be limited to the extent of about 5-15°. Comparison ofFIGS. 7 and 8 shows such maximum pivoting in one direction by an arc ofthe angle a; preferably, the pivoting allowed is not greater than about10°. When pivoting to the full extent is achieved, as depicted in FIG.8, the flat undersurface of the flange 87 of the insert 65 smoothlyabuts the frustoconical surface portion 93 of the stem.

Assembly of these components is essentially as hereinbefore described.Initial relative movement between the polymeric insert 65 and the head67 causes the insert to elastically, radially inwardly deform and thenexpand to its original shape and, if desired, form an interference fitonce fully within the pocket provided in the cavity 77 of the head.Subsequent relative axial movement between this subassembly and the stemelement 63 causes the spherical connector 73 to pass through the reduceddiameter entrance 91, deforming the polymeric material of the insert 65in a radially outward direction; the presence of the circumferentialrelief provided by the gap 88 accommodates such deflection in the regionof entrance ring portion to assure that stress upon the polymericmaterial remains within its elastic limits Then the entrance regionelastically snaps back into place, with the ball connector 73 seated inthe spherical cavity 89 of the insert. By proportioning the relief gap88 to be of sufficient size and the curved entrance region of the regionso that its diameter is between about 70% and 85% of the diameter of thesphere, and preferably about 75%±3%, it is assured that assembly can bereadily achieved and that inadvertent disassembly of the components ofthe implant 61 cannot occur once assembled; moreover, plastic flow ofthe chosen UHMWPE will not occur during assembly. Thus, the illustratedarrangement not only provides effective joinder between a somewhatbrittle pyrocarbon-coated graphite insert and a metal stem, but it alsoresults in a strengthened composite unit which is bi-polar, i.e.allowing a change in attitude of as much as angle a of arc in anydirection from a coaxial attitude where flange 87 of the insert 65 isperpendicular to the axis of the stem element 63.

Illustrated in FIGS. 9, 10 and 11 is another alternative embodimentshowing an implant 111 which, from its exterior appearance (FIG. 10),closely resembles the implant 11; however, the design of the polymericinsert and its interengagement with the connector of the stem element isdifferent, while the pyrocarbon-coated head is only slightly different.The implant 111 consists of an assembly of three components: a metalstem element 113, a polymeric insert 115, and a pyrocarbon-coatedgraphite head 117. As described previously, the metal alloy stem element113 includes an implantable stem portion 119, a circular flange 121, anda connector 123 which resembles the connector 23, being separated fromthe flange by a short neck 125. However, it includes a circumferentialgroove 126 which cooperates in the interengagement with the polymericinsert 115.

The head 117 is essentially the same as the head 17 as describedhereinbefore; it is an isotropic graphite substrate that is coated witha continuous coating of pyrocarbon having a thickness of at least about200 microns across the shallow, concave surface 127 at its axial end andthe lateral exterior surface 129. The carbon coating is continuous so asto cover the wall surface of an interior cavity 131 has a lateralinterior wall 133 of right circular cylindrical shape 133 thatterminates in a pair of transitional surfaces 135 a and 135 b which leadto the flat upper wall of the cavity and to the reentrant lip 137 whichforms the entrance to the cavity 131 of lesser diameter than the lateralwall surface 133.

In this embodiment, the polymeric insert 115 is in the shape of a sleevewhich fits entirely within the confines of the pyrocarbon-coated head117. It is again formed from a suitable polymeric material, preferablyUHMWPE, having suitable elastic properties as described hereinbefore.The sleeve is formed with a lateral surface 141 that is a surface formedby two spaced apart sections of a right circular cylinder thatterminates in a pair of transitional surfaces 143 a and 143 b that matchthe shape of the surfaces 135 a and b on the interior cavity of thepyrocarbon head and are juxtaposed therewith when assembled. The lateralsurface 141 is interrupted by a central annular groove or hollow 145which serves as a relief region as described hereinafter. The sleeve hasan interior surface 147, which is also that of a right circular cylinderand provides a central cavity to receive the connector 123, from whichsurface there is a central protruding circumferential locking flange 149of arcuate shape that serves to cause interengagement of the polymericinsert 115 and the connector 123 at the head of the stem element.

The circumferential groove 126 in the connector 123 is formed with adepth (H2 in FIG. 11) so as to receive the circumferential flange 149 ofthe insert and with a pair of parallel radial walls 151 that transitionbetween its outer cylindrical surface 123 and the base of the groove.The walls 151 are spaced apart by the distance W1 in FIG. 11.

A subassembly of the pyrocarbon-coated graphite head 117 and thepolymeric insert 115 is first made, as previously described, by relativemovement of the two so that the insert enters through the narrowedentrance provided by the protruding lip 137. The arcuate transitionalsurface 143 a, at the end of the insert, serves as a tapered lead-insurface that begins the radially inward elastic deformation of thegenerally tubular insert 115 as it moves past the narrower diameter ofthe lip 137. Once fully in place within the cavity 131 of the head, itreturns to its original shape. The insert is preferably proportioned soas to create an interference fit with the lateral surface 133 of thecavity of the head, with the two transitional surfaces 143 a and bjuxtaposing with the facing surfaces 135 a and b of the head cavity.

Next, relative movement between the stem element 113 and the subassemblycauses the connector 126 to slide past the pyrocarbon lip 137 where somevery slight clearance is provided and to likewise slide through theinitial section of the interior cavity surface 147 of the insert, untila chamfered lead-in section 155 at the end of the connector engages theprotruding circumferential flange 149 of the insert. At this point, themiddle region of the confined polymeric insert 115 is forced radiallyoutward in elastic deformation by the lateral surface 123 of theconnector. However, the relief region provided by the annular groove orhollow 145, that is at the same axial location as the protruding lockingflange 149, provides a region into which the polymeric material can movewithout undergoing plastic flow and again should have a volume at leastabout 90% of that of the protruding flange. Further insertion of thestem element 113 completes the assembly when the circumferential flangeof the polymeric insert is seated in the circumferential groove 126 ofthe stem connector, as depicted in FIG. 11. Proportioning is preferablysuch that the width of the protruding flange 149 is about the same asthe dimension W1 of the groove 126, and that the depth H1 of the reliefgroove 145 is about the same as the thickness H2 of the protrudingflange (which is about equal to the depth of the connector groove 126).As can be seen in this FIGURE, the interengagement of thecircumferential flange 149 within the groove 126, with the sharp cornersof the groove wall 151 seated at opposite ends of the circumferentialprotruding flange 149, creates a tight locking engagement so thatinadvertent disassembly is no longer possible.

Although the invention has been described with regard to certainpreferred embodiments which constitute the best most presently known forconstructing the invention, it should be understood that various changesand modifications may be made without departing from the scope of theinvention, which is set forth in the claims appended hereto. Althoughthe illustrated prostheses all illustrate an implant where the axis ofthe head is coaxial with the axis of the stem, it should be clear thatthis is not a requirement and that the head for a metacarpal phalangealjoint, for example, which might be preferably at an angle to the axis ofthe stem, could utilize the same assembly arrangement as illustratedherein. Likewise, it is not a requirement that the pyrocarbon head beclosed at its top; a head could be more in the form of a sleeve fromwhich the end of the stem element would protrude; head surface characteris simply dictated by the desired articular surface of the resultantimplant. Moreover, the stem element need not include a circumferentialflange, although such is preferred as it provides a positive locationfor placement of the implant in the bone being repaired. Although thehead for the implant is described as preferably being made frompyrocarbon-coated, isotropic graphite, particularly when the implant isto be used at a location where there is articulation, the invention isalso advantageous for the assembly of other crystalline, nonmetallicbrittle material heads to a metal stem, for example, ceramic heads thatlikewise are brittler and have significant differences in physicalproperties from metal alloy stems. Accordingly, the method of forming acomposite implant, including a high tensile strength metal alloy stemelement and a crystalline, brittle head, particularly one having anarticular surface, opens up the opportunity for prosthesis design totake advantage of desired features of materials for both head and stemconstruction. Moreover, the method of joinder of two such components ofdiffering physical properties that, in addition, enhances theoperational character of the head and renders the method particularlyvaluable. There may also be variations in the cross-sectional geometryof the polymeric insert circumferential flange; instead of the flangehaving a cross-section of a circle, other suitable cross-sections may beused, e.g. trapezoidal or triangular. It is also possible to reverse theinterference snap lock mechanism so that the polymeric insert is firstassembled on the metal stem component to form the initial subassembly.This subassembly would then be inserted into the head cavity, with thearrangement being such that a radially outwardly protrudingcircumferential flange is radially inwardly deformed until it reaches anappropriately shaped groove or pocket located in the interior wall ofthe head component, where the flange snaps outward into a lockinginterengagement.

Particular features of the invention are emphasized in the claims whichfollow.

1. A prosthetic implant for implantation into a resected bone, whichimplant comprises: a metal stem element which has a connector at one endthat is shaped with a reentrant region of reduced dimension and animplantable stem portion at the opposite end, said connector being asection of a sphere at least about 10% greater than a hemisphere, anintegral one-piece polymeric insert which has a central cavity thatreceives said connector, a head formed of brittle crystalline materialhaving an articular surface and an interior cavity being proportioned toreceive said polymeric insert, said head cavity having an entrance of asize such that at least a portion of said polymeric insert mustelastically deform inward to enter said cavity, said insert being formedwith means for interengaging with said connector which requires radiallyoutward deformation of at least a portion of said insert to permitentrance of said connector into said central cavity and lock said insertand said connector in engagement, and means providing a circumferentialrelief region into which said portion of said polymeric insert canelastically deform to facilitate final assembly, said integral polymericinsert being made of polymeric material having an elasticity, such that(a) it can deform radially inward sufficient to facilitate its entryinto said head cavity and then return to shape, (b) it can deformradially outward to facilitate assembly with said connector and thenreturn to shape, and (c) once assembled with both said head and saidconnector, disassembly cannot inadvertently occur.
 2. The implant ofclaim 1 wherein said polymeric insert has an exterior lateralcylindrical wall of circular cross-section and a tapered end region tofacilitate its entry into said entrance of said head cavity, whichentrance is defined by an inwardly protruding lip.
 3. The implant ofclaim 2 wherein said connector is joined to the remainder of said metalstem element by a neck portion of reduced dimension.
 4. The implant ofclaim 3 wherein said stem element has a radial flange of generallycircular shape located between said neck portion and said implantablestem portion.
 5. The implant of claim 4 wherein said polymeric insertcentral cavity has a spherical interior surface and wherein saidpolymeric insert has an entrance region leading to said central cavitythat has a diameter between about 70% and about 85% of the diameter ofsaid central cavity.
 6. The implant of claim 5 wherein said neck portionof said stem element has a length such that a subassembly of said headand said polymeric insert can pivot on said connector in any directionthrough an arc of between about 5° and about 15°.
 7. The implant ofclaim 2 wherein said head is made of isotropic crystalline graphite, allof the surfaces of which are coated with a continuous coating ofpyrocarbon.
 8. The implant of claim 7 wherein said head cavity and saidinsert have matching cylindrical surfaces, and the diameter of saidcylindrical outer surface of said polymeric insert is such that assemblyof said insert within said head creates an interference fit at saidcylindrical surfaces.
 9. The implant of claim 8 wherein said polymericinsert is made of ultrahigh molecular weight polyethylene which meetsASTM Standard F648.
 10. A prosthetic implant for implantation into aresected proximal end of the radius, which implant comprises: a metalstem element which has a connector at one end having a spherical surfacewith a reentrant region of reduced dimension between it and an oppositeend that is implantable in the radius, an integral one-piece polymericinsert which has a central cavity that receives said connector and aflange that circumscribes an entrance to said central cavity, and a headhaving an exterior articular surface and an interior cavity of circularcross section proportioned to receive said insert, said head comprisingan isotropic crystalline graphite substrate having interior and exteriorpyrocarbon surfaces, said head interior cavity having an entrance formedby a reentrant entrance lip of a lesser inner diameter than said headinterior cavity and being of a size such that said polymeric insert mustelastically deform radially inward to enter said interior cavity, saidpolymeric insert central cavity having an entrance region of a sizesmaller than said connector and having an outer groove which receivessaid head entrance lip and is proportioned so that an annular reliefregion remains in said groove when said insert is assembled with saidhead, and said integral polymeric insert being made of polymericmaterial having an elasticity, such that (a) it can deform radiallyinward sufficient to facilitate its entry into said head cavity andreturn to shape, (b) its entrance region can deform radially outward tofacilitate assembly with said connector, and (c) once assembled withboth said head and said stem connector, disassembly cannot inadvertentlyoccur.
 11. The implant of claim 10 wherein said polymeric insert has anexterior lateral cylindrical wall of circular cross-section and atapered end region to facilitate its entry into said entrance of saidhead cavity, which entrance is defined by an inwardly protruding lip.12. The implant of claim 11 wherein said connector is joined to theremainder of said metal stem element by a neck portion of reduceddimension.
 13. The implant of claim 12 wherein said stem element has aradial flange of generally circular shape located between said neckportion and said implantable stem portion.
 14. The implant of claim 13wherein said polymeric insert central cavity has a spherical interiorsurface and wherein said polymeric insert has an entrance region leadingto said central cavity that has a diameter between about 70% and about85% of the diameter of said central cavity.
 15. The implant of claim 14wherein said neck portion of said stem element has a length such that asubassembly of said head and said polymeric insert can pivot on saidconnector in any direction through an arc of between about 5° and about15°.
 16. The implant of claim 11 wherein said head is made of isotropiccrystalline graphite, all of the surfaces of which are coated with acontinuous coating of pyrocarbon.
 17. The implant of claim 16 whereinsaid head cavity and said insert have matching cylindrical surfaces, andthe diameter of said cylindrical outer surface of said polymeric insertis such that assembly of said insert within said head creates aninterference fit at said cylindrical surfaces.
 18. The implant of claim17 wherein said polymeric insert is made of ultrahigh molecular weightpolyethylene which meets ASTM Standard F648.
 19. A method for forming aprosthetic implant, which method comprises: providing a metal alloy stemelement which has a connector at one end having a spherical surface thatis shaped with a region of reduced diametric dimension intermediate ofthe other end, providing a head of crystalline, brittle material havingan interior cavity formed with an entrance of reduced diameter,providing an integral polymeric insert that is proportioned to seatwithin the interior cavity in the head, which insert has a centralspherical cavity that is proportioned to receive the connector at theend of the stem element, forming a subassembly by mating the polymericinsert with the head by insertion of the polymeric insert through theentrance into the head interior cavity in a manner in which thepolymeric material deforms elastically radially inward to facilitate itsentry and returns to shape within said the interior cavity, and thencompleting the prosthetic implant by mating said subassembly with thestem element by inserting the connector into the central cavity withinthe polymeric insert which causes the polymeric material to elasticallydeform radially outward which deformation is accommodated by acircumferential relief region in the polymeric insert, and then toreturn to a configuration having an interior dimension that thereafterprevents inadvertent disassembly of said subassembly from the stemelement.